As is known, the use of containers made of paper, especially for containing comestible products, is very wide-spread on the market.
It is further known that some types of containers require the use of lids or is covering elements able to maintain the product insulated and container more efficiently, if not actually sealed (sealed from air and liquids) internally of the container compartment.
Consider for example beakers for containing soft drinks, such as orangeade, or sweet drinks, coffee.
With the aim of avoiding spillage of liquids, or contamination thereof, lids made of moulded plastic material have been realised, which can grip by snap-fitting to the reinforced edge of the paper beaker.
It is clear that this type of product, though being widely available on the market, is prey to some drawbacks connected in particular to the disposal of different materials (paper and plastic) internally of a same product.
Further, the sealing performances of the container are not high, due to the type of coupling realised. For example, if the beaker is crushed the plastic lid comes away.
With the aim of at least possibly obviating the drawbacks, GB patent 688545 describes the realisation of containers and lids made of paper material, wherein the container and lid are screwable at an external threading of the container mouth.
However the realisation of the external threading is complex, as it is necessary to work by crushing and deformation of the neck of the container in a way which is difficult to control and structurally not very resistant.
Also, patent GB643674 relates to an improvement for closing lids for bottles illustrates a structure in which the paper material lid, specially threaded, is extremely complex and defined by a plurality of flat superposed paper layers, glued and specially deformed.
It is obvious that even this type of product is complex and expensive to realise, as well as not very reliable.
A further improvement relating to paper material container and their threaded closure elements is described in document GB 428909 which illustrates a beaker structure in which the upper portion is threaded such as to receive an upturned is lid whose corresponding threading is, in use, arranged internally of the container containing chamber of the beaker, as is clearly illustrated in the figures appended to the description.
Also with reference to the above patent, there are some drawbacks in particular connected to the requisite of sealing which the threaded paper material lids should have, but which they do not succeed in guaranteeing.
In particular, document GB 468161 addresses this problem, signalling the difficulty of realising threading on the neck of the container and on the lid that actually substantially coincide, such as to guarantee optimal seal of the fluid in the container.
With the aim of obviating the cited drawback, document GB 468161 describes realising threading on the container and the lids which are slightly different, such that during the stage of fitting the lid on the container, a further interference force is generated which improves the seal of the container.
It is however clear that this type of solution leads to the need to realise two different devices, of which the first has the task of realising the threading on the paper container and the second has the task of realising the different threading on the lid, in a controlled and different way.
Each type of container comprises the realisation of machines dedicated to the above-evidenced aims.
From the point of view of the production methodologies of the containers with a lid made of paper material, sole mention is made here of GB patent GB2382873, which illustrates a method for producing a threaded lid.
In particular the apparatus used exploits the presence of a plurality of expandable sectors positioned internally of the structure of the lid to be realised, which move radially and impress on the internal surface of the lateral wall of the lid a spiral groove.
It should be noted however that the methodology of the above GB patent has the drawback of realising interrupted threads.
In fact, the expansion of the male-threaded angular sectors internally of the lid is necessarily leaves undeformed zones on the lid, due to the necessary stresses when opening.
This leads to interruptions in the shape of the thread which consequently generate de-alignments of the lid during the stage of screwing and/or causing deformation stress on the lid itself such as to cause the lid to lose grip on the container.